Polygon Mirror

ABSTRACT

A polygon mirror that includes a top surface, a bottom surface, and a plurality of reflective surfaces disposed between the top surface and the bottom surface. Each reflective surface of the plurality of reflective surfaces forms an angle θ with an adjacent reflective surface. Additionally, each reflective surface of the polygon mirror has an RMS surface roughness of about 1.5 nm or less.

This application claims the benefit of priority U.S. Provisional PatentApplication Ser. No. 63/175,810 filed on Apr. 16, 2021, the content ofwhich is relied upon and incorporated herein by reference in itsentirety.

FIELD OF THE DISCLOSURE

The present disclosure is generally directed to a polygon mirror, andmore particularly relates to a polygon mirror with a low surfaceroughness.

BACKGROUND OF THE DISCLOSURE

Light detection and ranging (“lidar”) is used to measure a distance toan object by targeting the object with a light source and measuring thetime for the reflected light to reach a receiver. This technology iscommonly used in control and navigation systems for autonomous cars.However, the technology has a vast number of applications including thecreation of topographical maps, such as for use in agricultural andforestry. The light source of a traditional lidar system is a laser,which emits light with infrared, visible, or ultraviolet wavelengthstoward an object. The light is then reflected from the object and, insome systems, the distance to the object is determined based upon thetime for the light to be reflected back to the lidar system.

A traditional lidar system houses a scanner that includes a polygonmirror with reflective surfaces. The polygon mirror rotates so thatlight from the light source reflects off the different reflectivesurfaces of the polygon mirror. The light is reflected outward, awayfrom the lidar system, so that it scans an area for objects.

SUMMARY OF THE DISCLOSURE

Traditional polygon mirrors are formed of a metal, such as aluminum,which can be easily machined using a diamond-turning process. However,during such a process, the diamond-turning tools can leave marks orscratches on the polygon mirror, which can cause unwanted scattering oflight within a lidar system.

An object of the present disclosure is to provide polygon mirrors withlow surface roughness. Thus, the polygon mirrors disclosed herein areformed of a material that can be easily finished to have high surfacequality with low surface roughness. In some embodiments, the material isa glass, glass ceramic, or ceramic. Furthermore, the polygon mirrorsdisclosed herein have a lower density, as compared with the traditionalpolygon mirrors, thus requiring less energy for their rotation in alidar system.

Aspects of the present disclosure include a polygon mirror comprising atop surface, a bottom surface, and a plurality of reflective surfacesdisposed between the top surface and the bottom surface. Each reflectivesurface of the plurality of reflective surfaces forms an angle θ with anadjacent reflective surface. Additionally, each reflective surface hasan RMS surface roughness of about 1.5 nm or less.

Although many different embodiments are listed, the embodiments mayexist individually or in any combination as possible. Hereinafterexemplary embodiments are shown and described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a lidar system, according toembodiments of the present disclosure;

FIG. 2A is a schematic diagram illustrating an exemplary polygon mirror,according to embodiments of the present disclosure;

FIG. 2B is another schematic diagram illustrating an exemplary polygonmirror, according to embodiments of the present disclosure;

FIGS. 3A-3D are schematic diagrams illustrating an exemplary polygonmirror, according to embodiments of the present disclosure;

FIG. 3E is a schematic diagram illustrating exemplary coatings on thepolygon mirror, according to embodiments of the present disclosure;

FIG. 4A is a schematic diagram illustrating a process to produce apolygon mirror, according to embodiments of the present disclosure;

FIG. 4B is another schematic diagram illustrating a process to produce apolygon mirror, according to embodiments of the present disclosure;

FIG. 5A is an image of a surface profile of a traditional polygonmirror;

FIG. 5B is an image of a surface profile of an exemplary polygon mirror,according to embodiments of the present disclosure; and

FIG. 6 is a plot of power spectral densities of a traditional polygonmirror and an exemplary polygon mirror.

DETAILED DESCRIPTION

Additional features and advantages of the disclosure will be set forthin the detailed description which follows and will be apparent to thoseskilled in the art from the description, or recognized by practicing thedisclosure as described in the following description, together with theclaims and appended drawings.

As used herein, the term “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.For example, if a composition is described as containing components A,B, and/or C, the composition can contain A alone; B alone; C alone; Aand B in combination; A and C in combination; B and C in combination; orA, B, and C in combination.

In this document, relational terms, such as first and second, top andbottom, and the like, are used solely to distinguish one entity oraction from another entity or action, without necessarily requiring orimplying any actual such relationship or order between such entities oractions.

It will be understood by one having ordinary skill in the art thatconstruction of the described disclosure, and other components, is notlimited to any specific material. Other exemplary embodiments of thedisclosure disclosed herein may be formed from a wide variety ofmaterials, unless described otherwise herein.

It is also important to note that the construction and arrangement ofthe elements of the disclosure, as shown in the exemplary embodiments,is illustrative only. Although only a few embodiments have beendescribed in detail in this disclosure, those skilled in the art whoreview this disclosure will readily appreciate that many modificationsare possible (e.g., variations in sizes, dimensions, structures, shapesand proportions of the various elements, values of parameters, mountingarrangements, use of materials, colors, orientations, etc.) withoutmaterially departing from the novel and nonobvious teachings andadvantages of the subject matter recited. For example, elements shown asintegrally formed may be constructed of multiple parts, or elementsshown as multiple parts may be integrally formed, the operation of theinterfaces may be reversed or otherwise varied, the length or width ofthe structures, and/or members, or connectors, or other elements of thesystem, may be varied, and the nature or number of adjustment positionsprovided between the elements may be varied. It should be noted that theelements and/or assemblies of the system may be constructed from any ofa wide variety of materials that provide sufficient strength ordurability, in any of a wide variety of colors, textures, andcombinations. Accordingly, all such modifications are intended to beincluded within the scope of the present disclosure. Othersubstitutions, modifications, changes, and omissions may be made in thedesign, operating conditions, and arrangement of the desired and otherexemplary embodiments without departing from the spirit of the presentdisclosure.

Reference will now be made in detail to the present preferredembodiments of the disclosure, examples of which are illustrated in theaccompanying drawings. Whenever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Referring now to FIG. 1, an exemplary lidar system 10 is shown, whichincludes a light source 20, an optical mirror 30, and a polygon mirror40. In some embodiments, light source 20 is a laser configured to emitoutput light 22 with infrared, visible, or ultraviolet wavelengths. Forexample, light source 20 emits output light 22 having a wavelength fromabout 800 nm to about 1600 nm, or from about 800 nm to about 950 nm, orabout 1550 nm. Light source 20 may be a pulsed laser and/or a laserdiode.

Output light 22 from light source 20 is then reflected by optical mirror30 and onto polygon mirror 40, where the light is reflected again bypolygon mirror 40 and directed away from lidar system 10. Morespecifically, output light 22 is reflected by polygon mirror 40 adistance away from lidar system 10 and towards one or more objects. Itis noted that the light reflected by polygon mirror 40 is containedwithin a field of view (FOV). In some embodiments, FOV has dimensions ofabout 120° by about 30° so that lidar system 10 is able to scan andcapture objects within a 120° range in a lengthwise direction and a 30°range in a widthwise direction. However, the FOV can more or less thanthis range of about 120° by about 30°. The light reflected by polygonmirror 40 then reaches an object 55 within the FOV. As discussed furtherbelow, the light that is reflected from object 55 (reflected light 24)is directed from polygon mirror 40 to first mirror 30, where it is thenreflected to a receiver 60.

Optical mirror 30 may be a pivoting mirror such as a galvo mirror. Insome embodiments, optical mirror 30 rotates about an axis that isorthogonal to an axis of rotation of polygon mirror 40. As discussedabove, optical mirror 30 reflects output light 22 towards a reflectivesurface of polygon mirror 40 and reflects reflected light 24 towardsreceiver 60.

Receiver 60 is configured to receive reflected light 24 and produce acorresponding electrical signal, such as an electrical current orvoltage pulse. The electrical signal is then sent to a controller (notshown) for processing. More specifically, the controller determines thetime of flight of the light (output light 22 plus reflected light 24),which is then used to determine the distance of object 55 from lidarsystem 10. For example, if the time of flight of the light is determinedto be 100 ns, the controller may calculate the distance of object 55from lidar system 10 to be 15 m. Receiver 60 may comprise aphotoreceiver, optical receiver, optical sensor, detector,photodetector, and/or optical detector.

In some embodiments, lidar system 10 comprises two light sources 20(first and second light sources) and two optical mirrors 30 (first andsecond optical mirrors) such that the first light source 20 directsoutput light 22 to the first optical mirror and the second light sourcedirects output light 22 to the second optical mirror. The first opticalmirror then reflects the light onto a first reflective surface ofpolygon mirror 40 and the second optical mirror reflects the light ontoa second reflective surface of polygon mirror 40.

Although not shown in FIG. 1, lidar system 10 may include additionaloptical components such as, for example, lenses, mirror, filters, beamsplitters, and/or polarizers. These components are configured to directand modify output light 22 and/or reflected light 24.

Polygon mirror 40 is a rotating prismatic or pyramidal member withmultiple reflective surfaces. As shown in FIG. 1, a shaft of motor 41 iscoupled to polygon mirror 40 to provide rotation of polygon mirror 40.FIG. 2A shows a first exemplary embodiment of polygon mirror 40 in whichthe mirror comprises a top surface 42, a bottom surface 43, andreflective surfaces 44. In this embodiment, polygon mirror 40 has sixreflective surfaces 44. However, it is also contemplated that polygonmirror 40 can have more or less reflective surfaces 44. For example,polygon mirror 40 may be triangular-shaped with three reflectivesurfaces 44, or square-shaped with four reflective surfaces 44. In otherembodiments, polygon mirror 40 comprises 5 or more reflective surfaces,or 8 or more reflective surfaces, or 10 or more reflective surfaces.

In the embodiment of FIG. 2A, reflective surfaces 44 are each orientatedat an angle θ with regard to their adjacent reflective surfaces. In thisembodiment, each reflective surface 44 forms an angle θ of 60° with anadjacent reflective surface. In embodiments that comprise threereflective surfaces 44, each reflective surface 44 forms an angle θ of120° with an adjacent reflective surface. In embodiments that comprisefour reflective surfaces 44, each reflective surface 44 forms an angle θof 90° with an adjacent reflective surface. Furthermore, in embodimentsthat comprise five reflective surfaces 44, each reflective surface 44forms an angle θ of 72° with an adjacent reflective surface. Angle θ maybe in a range from about 60° to about 120°. Although reflective surfaces44 are shown as flat, planar surfaces in FIG. 2A, it is alsocontemplated that the surfaces may be rounded and curved. For example,one or more reflective surfaces 44 may be concave or convex.

Reflective surfaces 44 are each oriented to provide a facet-to-facetangular variance of about 30 arc-seconds or less, or about 20arc-seconds or less, or about 10 arc-seconds or less, wherein thefacet-to-facet angular variance is the variation of the normal angles(Ψ) amongst the reflective surface. As shown in FIG. 2B, the normalangles (Ψ) are each formed between lines perpendicular to the facet ofeach reflective surface 44. In some embodiments, the facet-to-facetangular variance is within a range of about 5 arc-seconds to about 30arc-seconds, or about 10 arc-seconds to about 20 arc-seconds.

As shown in FIG. 2A, polygon mirror 40 comprises an internal opening 45for coupling with motor 41.

FIGS. 3A-3D depict another exemplary embodiment of polygon mirror 40 inwhich the mirror comprises four reflective surfaces 44. FIG. 3A shows atop, perspective view of polygon mirror 40, FIG. 3B shows a bottom viewof polygon mirror 40, FIG. 3C shows a side view of polygon mirror 40,and FIG. 3D shows a cross-sectional view of polygon mirror. In thisembodiment, reflective surfaces 44 form a 90° angle with adjacentreflective surfaces. Polygon mirror 40 is shown as forming a square inFIGS. 3A-3C such that top surface 42 and bottom surface 43 are eachsquare-shaped. However, it is also contemplated that the polygon mirrors40 disclosed herein may have a rectangular-shape. A length L of polygonmirror 40 may range from about 20 mm to about 80 mm, or from about 30 mmto about 60 mm. A width W of polygon mirror 40 may also range from about20 mm to about 80 mm, or from about 30 mm to about 60 mm. Additionally,a thickness T of polygon mirror may range from about 5 mm to about 30mm, or from about 10 mm to about 20 mm.

It is also contemplated that the corners between adjacent reflectivesurfaces 44 may be rounded or chamfered. In other embodiments, thecorners between adjacent surfaces 44 may be textured to include, forexample, grooves or ribs.

As shown in FIG. 3D, a pyramidal error of reflective surfaces 40 ismeasured as the deviation between an angle formed by lines X and Yamongst each reflective surface 40 of polygon mirror 40. It is notedthat line Y in FIG. 3D is the outer facet of a reflective surface 44. Insome embodiments, polygon mirror 40 has a pyramidal error of about 70arc-seconds or less, or about 60 arc-seconds or less, or about 50arc-seconds or less, or about 20 arc-seconds or less, or about 10arc-seconds or less, or about 5 arc-seconds or less.

The reflective coating on reflective surfaces 44 may be disposed on theentirety of each reflective surface 40 (i.e., for the full width of thesurface between top surface 42 and bottom surface 43) or disposed onless than the entire surface for one or more reflective surfaces 44.FIG. 3C shows an embodiment in which the entire reflective surface 44 iscovered with a reflective coating 48. The reflective coating can be anymaterial suitable to reflect light in the IR, NIR, and/or visiblewavelengths. The reflective coating may comprise, for example, aluminum,sapphire, gold, silver, chrome, copper, nickel, titanium, orcombinations thereof.

As shown in FIG. 3E, in some embodiments, reflective coating 48 maycomprise an additional layer of a blocking or absorbing coating 49.Coating 49 may be disposed on an outward surface of reflective surfaces44, such that coating 49 is disposed between reflective surfaces 44 andreflective coating 48. Furthermore, coating 49 may be configured toremove any stray infrared light by absorption. In some embodiments,coating 49 comprises a metal such as, for example, Al, Au, Ag, or Cr. Inother embodiments, coating 49 comprises Si (amorphous orpolycrystalline) or CrON. It is also contemplated that coating 49comprises a combination of one or more of these materials.

In some embodiments, the reflective coating comprises dielectric layersof alternating layers of low and high refractive index materials. Thematerials with the low refractive index may have a refractive index inthe range of about 1.35 to about 1.5 and may comprise, for example,MgF₂, BaF₂, and SiO₂. The materials with the high refractive index mayhave a refractive index of about 1.9 to about 3.8 and may comprise, forexample, SiN (Si₃N₄), Si, Ta₂O₅, Ta₂O₂, TiO₂, Pr₂O₃, Nb₂O₃, HfO₂, Al₂O₃,Nb₂O₅, ZrO₂, and Y₂O₃.

The reflective coatings have a reflectance of about 85% or more, orabout 90% or more, or about 95% or more, or about 99% or more across theIR, NIR, and visible wavelength spectrum. The thickness of thereflective coating is in a range from about 1 micron to about 10microns, or about 3 microns to about 5 microns.

Polygon mirror 40 may be comprised of glass, glass ceramic, ceramic, ormetal. Preferred materials are glass and glass ceramics, as discussedfurther below, and include, for example, silicate glass, analuminosilicate glass, alkali aluminosilicate glass, alkalinealuminosilicate glass, borosilicate glass, boro-aluminosilicate glass,alkali aluminoborosilicate glass, alkaline aluminoborosilicate glass,soda-lime glass, fused quartz (fused silica), or other types of glass.Exemplary glass materials include, but are not limited to, high purityfused silica HPFS® sold by Corning Incorporated of Corning, N.Y. underglass codes 7980, 7979, and 8655, and EAGLE XG® boro-aluminosilicateglass also sold by Corning Incorporated of Corning, N.Y. Other glasssubstrates include, but are not limited to, ultra-low expansion ULE®glass, Lotus™ NXT glass, Iris™ glass, WILLOW® glass, GORILLA® glass,VALOR® glass, Vycor™ glass, or PYREX® glass sold by Corning Incorporatedof Corning, N.Y. In some embodiments, polygon mirror 40 is comprised offloat glass, such as soda lime glass. In yet other embodiments, polygonmirror 40 is comprised of silica glass with 80 wt. % or more of silica,or 85 wt. % or more of silica, or 90 wt. % or more of silica, or 95 wt.% or more of silica, or 99 wt. % or more of silica.

Exemplary glass ceramics include, for example, lithium disilicate,nepheline, beta-spodumene, and beta-quartz. Exemplary commerciallyavailable materials include, for example, Macor® and Pyroceram® sold byCorning Incorporated of Corning, N.Y.

The material of polygon mirror 40 should be capable of achieving the lowsurface roughness disclosed further below, regardless of thetransparency of the material.

The polygon mirrors 40 disclosed herein have a lower density comparedwith the traditional aluminum polygon mirrors. More specifically, thedensity of polygon mirror 40 has a density of less than about 2.7 g/cc,or about 2.5 g/cc or less, or about 2.2 g/cc or less, or about 2.0 g/ccor less, or about 1.8 g/cc or less. In some embodiments, the density ofpolygon mirror 40 is about 2.2 g/cc, or about 2.3 g/cc. This is muchless than the traditional aluminum polygon mirrors, which have a densityof 2.7 g/cc. By having a lower density, the polygon mirrors 40 disclosedherein are lighter and, thus, require less energy to rotate. Therefore,they have a reduced power consumption in lidar system 10, whichadvantageously lowers costs. Furthermore, the lower density of polygonmirrors 40 enables a faster rotating start-up rate from a state of rest.

As discussed above, traditionally polygon mirrors were formed ofaluminum, such as 6061 alloy, because they could be machined using adiamond-turning process. However, the aluminum material is prone toscratching or marking by the diamond-turning tools, which causesunwanted scattering of the light in a lidar system. The polygon mirrors40 disclosed herein need not be made by such diamond-turning processes,thus providing polygon mirrors without such scratching or marking. Asdiscussed further below, due to the material that they are comprised of,the polygon mirrors 40 disclosed herein may be made by such processes aspolishing and/or molding, which do not cause such scratches and marks onthe produced polygon mirror. Thus, the polygon mirrors 40 disclosedherein have a smoother finish with a lower surface roughness, ascompared with the traditional polygon mirrors formed of aluminum. Due totheir lower surface roughness, the polygon mirrors 40 disclosed hereinhave less unwanted scattering of light. Therefore, more light isreflected onto object 55 and input into receiver 60, which provides amore accurate determination of the distance of object 55 from lidarsystem 10. Furthermore, improving the surface roughness of the mirrorsalso improves the ratio of signal to noise in the detected light.

In some embodiments, each reflective surface 44 of polygon mirror 40 hasa low RMS (root mean square) surface roughness in a range from about 2.0nm or less, or about 1.5 nm or less, or about 1.0 nm or less, or about0.5 nm or less, or about 0.25 nm or less, or about 0.15 nm or less, orabout 0.10 nm or less, as measured by white light interferometry using aspatial frequency within a range from 5 mm⁻¹ to 1,000 mm⁻¹. In someembodiments, reflective surfaces 44 each have an RMS surface roughnessof about 0.45 nm, or about 0.43 nm, or about 0.42 nm. The RMS surfaceroughness is determined using equation (1):

$\begin{matrix}{S_{q} = \sqrt{\frac{1}{A}{\int_{A}{{Z^{2}\left( {x,y} \right)}dxdy}}}} & (1)\end{matrix}$

wherein Sq is the RMS surface roughness, A is the total measurement area(micrometers²), and Z is the measured residual height data array in thespace domain (nm), wherein Z is measured using white lightinterferometry using the spatial frequency within the range from 5 mm⁻¹to 1,000 mm⁻¹.

As discussed above, the polygon mirrors 40 disclosed herein can beproduced by polishing and/or molding processes. More specifically, thematerial of polygon mirror 40 is capable of being shaped and formed by apolishing and/or molding process. For example, the glass, glass ceramic,and ceramic materials disclosed above are configured to obtain highsurface quality (e.g., low surface roughness) through such polishingand/or molding processes. Thus, the polygon mirrors 40 disclosed hereindo not need to rely on a diamond-turning process to shape and form thepolygon mirrors.

Exemplary polishing processes may include a rough polish/grind followedby a fine polish/grind. In other embodiments, polygon mirrors 40 areshaped and formed using other chemical and/or mechanical processes, suchas, for example, a chemical mechanical planarization (CMP) process, anion beam material removal process, a dry plasma etching process, or awet etch process.

In one exemplary process, as shown in FIG. 4A, multiple polygon mirrors40 are stacked on top of each and polished simultaneously. For example,side A of the multiple polygon mirrors 40 are each polishedsimultaneously, followed by side B of the multiple polygon mirrors 40,then side C, followed by side D. In other embodiments, sides A and C ofthe multiple polygon mirrors 40 are polished together simultaneously ina double-sided polishing procedure, followed by the polishing of sides Band C simultaneously. Stacking the polygon mirrors 40 on top of each andpolishing one or more sides simultaneously with the other stackedpolygon mirrors, reduces time and resources, thus providing a moreefficient polishing procedure.

FIG. 4B shows another exemplary process to form multiple polygon mirrors40 simultaneously. In this exemplary process, at step 100, opening 106is first formed in blank 102. Opening 106 may be a cylindrical aperturewithin blank 102. Next, at step 110, blank 102 is segmented and dicedinto multiple units 122. Each unit 122 may represent a single polygonmirror 40. Then, at step 130, internal features of opening 106 aremodified and/or added. For example, a portion of opening 106 is enlargedso that opening 106 has a non-uniform diameter across its length. Atstep 140, the outer surfaces of units 122 are then polished. Each unit122 may be polished separately, or multiple units 122 may be stackedtogether and polished simultaneously, as discussed above. Step 140 mayproduce polished polygon mirrors 40 with a high surface quality. In thepolished polygon mirror 40, opening 106 may be configured for a shaft ofa motor to be disposed therethrough. It is also noted that the processshown in FIG. 4B may include all or less than of all of steps 100through 140.

FIGS. 5A and 5B show a comparison of surface profiles for a traditionalpolygon mirror formed of aluminum 6061 AA (FIG. 5A) and an exemplarypolygon mirror formed of HPFS® glass (FIG. 5B). The traditional polygonmirror was formed by a diamond-turning process. However, the exemplarypolygon mirror was formed by a polishing process as disclosed herein.The surface profiles of FIGS. 5A and 5B were measured using awhite-light interferometer. As shown in FIG. 5A, the diamond-turnedtraditional polygon mirror has a streaky surface profile, due to thetool marks from the diamond-turning process. The RMS surface roughnessof the traditional polygon mirror was determined to be 4 nm. Incomparison, the exemplary polygon mirror of FIG. 5B does not have astreaky surface profile. Furthermore, the RMS surface roughness of theexemplary polygon mirror in FIG. 5B was determined to be 0.43 nm, whichis a significant improvement from the traditional polygon mirror.

FIG. 6 shows the power spectral densities (PSD) for the polygon mirrorsof FIGS. 5A and 5B. Therefore, plot A in FIG. 6 represents thetraditional polygon mirror formed of aluminum 6061 AA (as also shown inFIG. 5A), and plot B of FIG. 6 represents the exemplary polygon mirrorformed of HPFS® glass (as also shown in FIG. 5B). Plot B in FIG. 6 has alower PSD over the specified frequency spectrum. It is noted that PSDcan be used to analyze surface roughness. PSD provides a representationof the amplitude of a surface's roughness as a function of the spatialfrequency of the roughness. Thus, plot B has a lower power spectraldensity over the specified frequency spectrum, as compared with plot A,and, therefore, also has a lower surface roughness. More specifically,the PSD of plot B is reduced by two orders of magnitude from the PSD ofplot A.

The polygon mirrors 40 disclosed herein can be used in other systemsthan a lidar system, such as, for example, bar code scanners, laserprinters, and laser markers.

While various embodiments have been described herein, they have beenpresented by way of example only, and not limitation. It should beapparent that adaptations and modifications are intended to be withinthe meaning and range of equivalents of the disclosed embodiments, basedon the teaching and guidance presented herein. It therefore will beapparent to one skilled in the art that various changes in form anddetail can be made to the embodiments disclosed herein without departingfrom the spirit and scope of the present disclosure. The elements of theembodiments presented herein are not necessarily mutually exclusive, butmay be interchanged to meet various needs as would be appreciated by oneof skill in the art.

It is to be understood that the phraseology or terminology used hereinis for the purpose of description and not of limitation. The breadth andscope of the present disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

What is claimed is:
 1. A polygon mirror comprising: a top surface and abottom surface; and a plurality of reflective surfaces disposed betweenthe top surface and the bottom surface, each reflective surface of theplurality of reflective surfaces forming an angle θ with an adjacentreflective surface, wherein each reflective surface has an RMS surfaceroughness of about 1.5 nm or less.
 2. The polygon mirror of claim 1,wherein the RMS surface roughness is about 1.0 nm or less.
 3. Thepolygon mirror of claim 2, wherein the RMS surface roughness is about0.5 nm or less.
 4. The polygon mirror of claim 1, wherein the polygonmirror is formed of a glass, glass ceramic, or ceramic material.
 5. Thepolygon mirror of claim 4, wherein the polygon mirror is formed ofsilica glass.
 6. The polygon mirror of claim 5, wherein the polygonmirror is formed of silica glass with 90 wt. % or more of silica.
 7. Thepolygon mirror of claim 5, wherein the polygon mirror is formed ofaluminosilicate glass, alkali aluminosilicate glass, alkalinealuminosilicate glass, borosilicate glass, boro-aluminosilicate glass,alkali aluminoborosilicate glass, alkaline aluminoborosilicate glass, orcombinations thereof.
 8. The polygon mirror of claim 5, wherein thepolygon mirror is formed of high purity fused silica glass.
 9. Thepolygon mirror of claim 1, wherein the polygon mirror has a density ofless than about 2.7 g/cc.
 10. The polygon mirror of claim 9, wherein thedensity is about 2.5 g/cc or less.
 11. The polygon mirror of claim 10,wherein the density is about 2.2 g/cc or less.
 12. The polygon mirror ofclaim 1, wherein the plurality of reflective surfaces each comprise areflective coating.
 13. The polygon mirror of claim 12, wherein thereflective coating is configured to reflect light in the IR, NIR, orvisible wavelengths.
 14. The polygon mirror of claim 13, wherein thereflective coating comprises aluminum, sapphire, gold, silver, chrome,copper, nickel, titanium, or combinations thereof.
 15. The polygonmirror of claim 13, wherein the reflective coating comprises dielectriclayers of alternating layers of low and high refractive index materials.16. The polygon mirror of claim 12, wherein the reflective coating has areflectance of about 99% or more across the IR, NIR, and visiblewavelength spectrum.
 17. The polygon mirror of claim 1, furthercomprising a blocking coating disposed on the reflective surfaces. 18.The polygon mirror of claim 17, wherein the blocking coating comprisesAl, Au, Ag, Cr, Si, CrON, or combinations thereof.
 19. The polygonmirror of claim 1, wherein the angle θ is in a range from about 60° toabout 120°.
 20. The polygon mirror of claim 19, wherein the angle θ isabout 90°.